The invention is especially, but not exclusively, applicable to the field of reaming hip-joint cavities (acetabulea) in the pelvis to a desired form, in preparation for inserting an acetabular cup. Therefore, the technical background to the invention will be described with specific reference to cutters or reamers for shaping acetabular cavities and the problems encountered when using prior-art cutters. A preferred embodiment of the reamer according to the invention is especially useful in connection with the type of acetabular cups disclosed in WO 95/17140. However, the invention may be used also in connection with the shaping of other sockets for receiving artificial joint prosthesis components.
One conventional type of reamer for shaping bone sockets comprises a cutting head, which in use is rotated about a rotational axis for removing material from the socket, and which cutting head is formed by a fully hemispherical wall defining an essentially closed internal chamber. A plurality of relatively small, outwardly projecting, bowl-shaped cutting elements, each defining a small cutting edge, are punched out from or otherwise formed in the hemispherical wall. This design is normally referred to as a "cheese grater". Each cutting element forms a narrow passage leading from the exterior hemispherical surface into the internal chamber of the cutting head. In order to avoid undesired interference phenomena during the reaming operation, the cutting elements are normally distributed in an asymmetrical pattern over the hemispherical surface. Especially, the cutting elements may be distributed along a spiral line as seen in the direction of the rotational axis.
A drawback of this conventional type of reamer having an essentially closed configuration and a plurality of small bowl-shaped cutting elements is that loose cut-off material (debris) will be present at the site of the cutting operation. This is a consequence of the essentially closed configuration of the cutting head and the fact that the bowl-shaped cutting members form only narrow passages for the cut-off material. The presence of loose material at the cutting site may lead to an inaccurate cavity preparation, whereby undesired gaps will be present between the reamed cavity surface and the subsequently inserted acetabular cup. Bone growths cannot bridge too large a gap, so that a portion of the cup surface will be too distant from bone to allow proper attachment. As a result, an optimal cup fixation cannot be obtained, especially in the case of uncemented prosthesis components, where such gaps may reduce initial and long term stability. Also, loose material will clog the reamer, causing an undesired excessive heat generation in the bone tissue, which may damage the bone-tissue viability and thereby reduce its capacity to form new bone. New bone formation is essential for obtaining osseointegration.
Another drawback of this prior-art reamer is that the cutting operation effected by the relatively small, bowl-shaped cutting elements will not result in a smooth cavity surface. Instead, each bowl-shaped cutting element creates circumferentially directed "macro"-grooves in the socket surface during the reaming operation.
A further drawback of this conventional-type reamer is that the asymmetrical distribution of the cutting elements may generate an unintentional "wobbling" of the reamer during rotation thereof. More specifically, as a result of the cutting elements not being symmetrically arranged over the hemispherical surface, the counter-forces generated at each cutting point may together lead to a situation where the cutting head is unequally loaded. The resultant load acting on the cutting head may therefore displace and/or tilt the axis of rotation, whereby the cavity will receive an undesired shape, such as an elliptical cross section.
The above-mentioned drawback related to the formation of circumferentially directed grooves may be reduced by replacing the relatively small bowl-shaped cutting elements with longer cutting edges extending in essentially radial planes along the hemispherical surface, as shown in U.S. Pat. No. 4,131,116. This document discloses a bone cutter for shaping a socket, such as a hip socket, comprising a cutting head located at one end of a rotatably driven shaft and having a fully hemispherical portion with a substantially closed exterior surface. The hemispherical portion defines an essentially closed, hollow internal chamber, and is provided with a number of slots connecting the exterior surface and the internal chamber. A cutting edge, raised slightly above the hemispherical surface, is provided at each slot for moving bone material from the socket into the internal chamber. In the disclosed embodiment, one cutting edge extends past the pole region, i.e. the point where the axis of rotation intersects the hemispherical surface, such that also the bottom of the socket will be reamed.
Resilient quick release means are arranged for connecting and disconnecting the cutting head and the shaft. A cylindrical portion integral with the base of the hemispherical portion is arranged for limiting outward movement of said resilient quick release means.
This prior-art document also describes a conventional reaming technique of starting with a cutting head of smaller radius and increasing the size of the cutting head in increments to enlarge the radius of the socket.
A drawback with the design disclosed in U.S. Pat. No. 4,131,116 is that are undesired "catching" or "biting" effect may occur between the cutting means and the cavity edge, especially each time the cutting head diameter is increased.
Another drawback inherent in cutting heads in the form of a full spherical dome is that it is difficult to guide the reamer precisely along the intended rotational axis without tilting the cutting head in the bone socket.
It is, therefore, an aim of the invention to provide a reamer by which the shaping or preparation of a bone socket can be performed more precisely.
A specific aim of the invention is to provide a reamer by which the above-mentioned "catching effect" is eliminated or at least substantially reduced.
A further aim of the invention is to provide a reamer which eliminates or at least substantially reduces the above-mentioned problem of cut-off material present at the cutting site between the reamer and the bone socket.
A yet further aim of the invention is to provide a reamer by which a smooth inner surface of the socket can be obtained.
Another further aim of the invention is to provide a reamer which is more easy to guide along a predetermined rotational axis during the reaming operation, in order to avoid tilting of the reamer relative to said axis.
It is also an aim of the invention to provide a method for implanting a prosthesis component in a bone socket, by which method the shaping or preparation of the bone socket can be performed more precisely, and to provide a kit for use in performing such a method.